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Handlers
========
Purpose
-------
Handlers are Erlang modules that represent a resource.
Handlers must process the request and send a reply. The nature of the
reply will vary between handlers.
Different kinds of handlers can be combined in a single module. This
allows a module to handle both websocket and long-polling code in a
single place, for example.
Protocol upgrades
-----------------
Cowboy features many different handlers: HTTP handlers, loop handlers,
websocket handlers, REST handlers and static handlers. All of them
have a common entry point: the `init/3` function.
By default, Cowboy considers your handler to be an HTTP handler.
To switch to a different protocol, like, for example, Websocket,
you must perform a protocol upgrade. This is done by returning
a protocol upgrade tuple at the end of `init/3`.
The following snippet upgrades the handler to `my_protocol`.
``` erlang
init(_Any, _Req, _Opts) ->
{upgrade, protocol, my_protocol}.
```
The `my_protocol` module will be used for further processing of the
request. It requires only one callback, `upgrade/4`.
@todo Describe `upgrade/4` when the middleware code gets pushed.
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